1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to such a computer tomography apparatus having the capability of preparing x-ray shadowgraphs.
2. Description of the Prior Art
Computer tomography systems are known in the art which have the capability of producing one or more x-ray shadowgraphs of an examination subject, in order to define the examination region before conducting the computer-tomographic examination. In such known systems, the patient is usually moved through the measurement opening, with the x-ray focus being held at a fixed angular position, and a shadowgraph, i.e., an x-ray projection image, is produced continuously or pulsed line-by-line. This conventional exposure technique for producing a shadowgraph involves the following problem.
Particularly given a fast scan sequence, known as dynamic CT and spiral CT, the processing speed of image processors, even high-performance CT image processors, is usually not adequate to reconstruct the CT tomograms in steps and to display such tomograms in real time on the monitor. This means that the examining attendant has no immediate visual control over the current exposure execution. The momentary slice position, and possible patient motions during the scan sequence, thus cannot be directly perceived. The shadowgraph obtained prior to conducting the CT examination will not include such image information.
It is known to reconstruct both computer tomograms and shadowgraphs for arbitrary projection directions from the data generated during scanning of a measurement volume, and to reproduce the shadowgraph in real time, as described in German OS 41 03 588.